Droplets that live fast, and die young

Blanche absinthe louche
A recent paper argues, and provides some experimental evidence for, droplets that as soon as they form, promptly head off to a region where they are unstable and so dissolve. The droplets are forming in what is sometimes called the ouzo effect, which is illustrated above. When water is added to ouzo (or similar aniseed-flavoured spirits like pastis, absinthe etc), the drink turns cloudy due to small droplets forming — these scatter light turning the drink cloudy. Above, the neat absinthe is on the left and and is clear, the drinks in the middle and on the right have added water and so are cloudy.
The droplets form because aniseed flavour comes from a molecule called anethole. Anethole dissolves in strong alcohol/water mixtures like neat ouzo, but is almost insoluble in water. So when water mixes with, and dilutes, the approximately 40% alcohol ouzo, the anethole comes out as tiny droplets.

If you just dilute ouzo at home you can’t see how these droplets form, but with better equipment, Hajian and Hardt, were able to see motion of these droplets (they still couldn’t see individual droplets as theirs were probably less than a thousandth of a millimetre across). The droplets were moving away from the more alcohol-dilute regions, and towards the alcohol-rich regions. In the alcohol-rich regions, the droplets dissolve and disappear. It looks like until the alcohol and water have completely mixed, there is a tendency for any droplets to head towards the alcohol-rich regions in the mixing liquid. There, if the alcohol concentration is high enough, the droplets dissolve. Droplets only form permanently when the water and alcohol have mixed and there are no alcohol-rich regions to dissolve them.

Droplets are held together by surface tension: the force that keeps drops spherical in shape. The surface tension of the droplets depends on the alcohol concentration — it is low at high alcohol concentrations — and ultimately if there is enough alcohol around the droplet the surface tension becomes zero, and then with nothing to hold the droplet together it will dissolve.

If a droplet is immersed in a gradient of concentration, then the surface tension of the back and front of the droplet will be different*. As surface tensions exert forces, this difference in surface tension from back to front creates an imbalance in these forces, and this imbalance creates flow around and inside the droplet. Although it is hard to visualise this, if you work through the fluid mechanics maths, these flows push the droplet in the direction of lower surface tension, which here is the direction of increasing alcohol concentration.

So, as soon as they form, the droplets move to their doom, in the alcohol-rich part of the liquid, until mixing is complete, and there are no alcohol-rich parts left. If you prepare yourself a diluted ouzo, absinthe etc, then I would guess this effect is happening, at least in the instants following you pouring in the water.

* This is called the Marangoni effect, see an earlier blog post on this (with link to YouTube movie).

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